Rapid O3 assimilations - Part 1: Background and local contributions to tropospheric O3 changes in China in 2015-2020

A single ozone (O₃) tracer mode was developed in this work to build the capability of the Goddard Earth Observing System model with Chemistry (GEOS-Chem) for rapid O₃ simulation. The single O₃ tracer simulation demonstrates consistency with the GEOS-Chem full chemistry simulation, with dramatic reductions in computational costs of approximately 91 %-94 %. The single O₃ tracer simulation was combined with surface and Ozone Monitoring Instrument (OMI) O₃ observations to investigate the changes in tropospheric O₃ over eastern China in 2015-2020. The assimilated O₃ concentrations demonstrate good agreement with O₃ observations because surface O₃ concentrations are 43.2, 41.8, and 42.1 ppb and tropospheric O₃ columns are 37.1, 37.9, and 38.0 DU in the simulations, assimilations, and observations, respectively. The assimilations indicate rapid rises in surface O₃ concentrations by 1.60 (spring), 1.16 (summer), 1.47 (autumn), and 0.80 ppb yr⁻¹ (winter) over eastern China in 2015-2020, and the increasing trends are underestimated by the a priori simulations. More attention is suggested to the rapid increases in the O₃ pollution in spring and autumn. We find stronger rises in tropospheric O₃ columns over highly polluted areas due to larger local contributions, for example, 0.12 DU yr⁻¹ (North China Plain) in contrast to −0.29 (Sichuan Basin) and −0.25 DU yr⁻¹ (southern China). Furthermore, our analysis demonstrated noticeable contributions of the interannual variability in background O₃ to the trends in surface O₃ (particularly in the summer) and tropospheric O₃ columns over eastern China in 2015-2020. This work highlights the importance of rapid simulations and assimilations to extend and interpret atmospheric O₃ observations.